I. Field
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for demodulating code division multiple access (CDMA) signals.
II. Description of the Related Art
In a wireless radiotelephone communication system, many users communicate over a wireless channel to connect to wireline telephone systems. Communication over the wireless channel can be one of a variety of multiple access techniques that allow a large number of users in a limited frequency spectrum. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
The CDMA technique has many advantages. An exemplary CDMA system is described in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” assigned to the assignee of the present invention.
In the '307 patent, a multiple access technique is disclosed where a large number of mobile telephone system users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations using CDMA spread spectrum communication signals. The base station-to-mobile station signal transmission path is referred to as the forward link and the mobile station-to-base station signal transmission path is referred to as the reverse link.
In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. Each base station provides coverage to a limited geographic area and links the mobile stations in its coverage area through a cellular system switch to the public switched telephone network (PSTN). When a mobile station moves to the coverage area of a new base station, the routing of that user's call is transferred to the new base station.
The CDMA modulation techniques discussed in the '307 patent and in U.S. Pat. No. 5,102,459 entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the present invention, mitigate the special problems of the terrestrial channel, such as multipath and fading. Instead of being an impediment to system performance, as it is with narrowband systems, separable multipaths can be diversity combined in a mobile rake receiver for enhanced modem performance. The use of a RAKE receiver for improved reception of CDMA signals is disclosed in U.S. Pat. No. 5,109,390, entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the present invention. In the mobile radio channel, multipath is created by reflection of the signal from obstacles in the environment, such as buildings, trees, cars, and people. In general the mobile radio channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over the time varying multipath channel, the received stream of pulses would change in time location, attenuation, and phase as a function of the time that the ideal impulse was transmitted.
The multipath properties of the terrestrial channel produce, at the receiver, signals having traveled several distinct propagation paths. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. As described in the '390 patent, the spread spectrum pseudonoise (PN) modulation used in a CDMA system allows different propagation paths of the same signal to be distinguished and combined, provided the difference in path delays exceeds the PN chip duration. If a PN chip rate of approximately 1 MHz is used in a CDMA system, the full spread spectrum processing gain, equal to the ratio of the spread bandwidth to the system data rate, can be employed against paths having delays that differ by more than one microsecond. A one microsecond path delay differential corresponds to a differential path distance of approximately 300 meters.
Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation factor. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than other received pulses.
Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. If, for example, an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than other received pulses. This can result in signal fading.
A fade occurs when multipath vectors are added destructively, yielding a received signal that is smaller than either individual vector. For example, if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB, a time delay of d with a phase shift of Q radians, and the second path has an attenuation factor of X dB, a time delay of d with a phase shift of Q+π radians, no signal would be received at the
As described above, in current CDMA demodulator structures, the PN chip interval defines the minimum separation two paths must have in order to be combined. Before the distinct paths can be demodulated, the relative arrival times (or offsets) of the paths in the received signal must first be determined. The demodulator performs this function by “searching” through a sequence of offsets and measuring the energy received at each offset. If the energy associated with a potential offset exceeds a certain threshold, a demodulation element, or “finger” may be assigned to that offset. The signal present at that path offset can then be summed with the contributions of other fingers at their respective offsets.
A method and apparatus of finger assignment based on searcher and finger energy levels is disclosed in U.S. Pat. No. 5,490,165, entitled “FINGER ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS,” assigned to the assignee of the present invention. In the exemplary embodiment, the CDMA signals are transmitted in accordance with the Telecommunications Industry Association TIA/EIA/IS-95-A, entitled “MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEM.” An exemplary embodiment of the circuitry capable of demodulating IS-95 forward link signals is described in detail in U.S. Pat. No. 5,764,592, entitled “MOBILE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS SYSTEM,” assigned to the assignee of the present invention. An exemplary embodiment of the circuitry capable of demodulating IS-95 reverse link signals is described in detail in U.S. Pat. No. 5,654,979, entitled “CELL SITE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM,” assigned to the assignee of the present invention.
FIG. 1 shows an exemplary set of signals from a base station arriving at the mobile station. It will be understood by one skilled in the art that FIG. 1 is equally applicable to the signals from a mobile station arriving at the base station. The vertical axis represents the power received on a decibel (dB) scale. The horizontal axis represents the delay in the arrival time of a signal due to multipath delays. The axis (not shown) going into the page represents a segment of time. The signals in the common plane traveled along different paths arriving at the receiver at the same time, but having been transmitted at different times.
In a common plane, peaks to the right were transmitted at an earlier time by the base station than peaks to the left. For example, the left-most peak spike 2 corresponds to the most recently transmitted signal. Each signal spike 2-7 has traveled a different path and therefore exhibits a different time delay and a different amplitude response.
The six different signal spikes represented by spikes 2-7 are representative of a severe multipath environment. Typical urban environments produce fewer usable paths. The noise floor of the system is represented by the peaks and dips having lower energy levels.
The task of the searcher is to identify the delay as measured by the horizontal axis of signal spikes 2-7 for potential finger assignment. The task of the finger is to demodulate one of a set of the multipath peaks for combination into a single output. It is also the task of a finger, once assigned to a multipath peak, to track that peak as it may move in time.
The horizontal axis can also be thought of as having units of PN offset. At any given time, the mobile station receives a variety of signals from a base station, each of which has traveled a different path and may have a different delay than the others. The base station's signal is modulated by a PN sequence. A local copy of the PN sequence is also generated at the mobile station. Also at the mobile station, each multipath signal is individually demodulated with a PN sequence code aligned to its received time offset. The horizontal axis coordinates can be thought of as corresponding to the PN sequence code offset that would be used to demodulate a signal at that coordinate.
Note that each of the multipath peaks varies in amplitude as a function of time, as shown by the uneven ridge of each multipath peak. In the limited time shown, there are no major changes in the multipath peaks. Over a more extended time range, multipath peaks disappear and new paths are created as time progresses. The peaks can also slide to earlier or later offsets as the path distances change when the mobile station moves relative to the base station. Each finger tracks these small variations in the signal assigned to it.
In narrowband systems, the existence of multipath in the radio channel can result in severe fading across the narrow frequency band being used. Such systems are capacity constrained by the extra transmit power needed to overcome a deep fade. As noted above, CDMA signal paths may be discriminated and diversity combined in the demodulation process.
Three major types of diversity exist: time diversity, frequency diversity, and space/path diversity. Time diversity can best be obtained by the use of repetition, time interleaving, and error correction and detection coding that introduce redundancy. A system may employ each of these techniques as a form of time diversity.
CDMA, by its inherent wideband nature, offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. The frequency selective fading that can cause a deep fade across a narrowband system's frequency bandwidth usually only affects a fraction of the frequency band employed by the CDMA spread spectrum signal.
The rake receiver provides path diversity through its ability to combine multipath delayed signals; all paths that have a finger assigned to them must fade together before the combined signal is degraded. Additional path diversity is obtained through a process known as “soft hand-off” in which multiple simultaneous, redundant links from two or more base stations can be established with the mobile station. This supports a robust link in the challenging environment at the cell boundary region. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled “SOFT HAND-OFF IN A CDMA CELLULAR TELEPHONE SYSTEM,” and U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention.
Both the cross-correlation between different PN sequences and the autocorrelation of a PN sequence, for all time shifts other than zero, have a nearly zero average value. This allows the different signals to be discriminated upon reception. Autocorrelation and cross-correlation require that logical “0” take on a value of “1” and logical “1” take on a value of “−1”, or a similar mapping, in order that a zero average value be obtained.
However, such PN signals are not orthogonal. Although the cross-correlation essentially averages to zero over the entire sequence length for a short time interval, such as an information bit time, the cross-correlation is a random variable with a binomial distribution. As such, the signals interfere with each other in much the same manner as if they were wide bandwidth Gaussian noise at the same power spectral density.
It is well known in the art that a set of n orthogonal binary sequences, each of length n, for n any power of 2 can be constructed (see Digital Communications with Space Applications, S. W. Golomb et al., Prentice-Hall, Inc., 1964, pp. 45-64). In fact, orthogonal binary sequence sets are also known for most lengths that are multiples of four and less than two hundred. One class of such sequences that is easy to generate is called the Walsh function; a Walsh function of order n can be defined recursively as follows:
                              W          ⁡                      (            n            )                          =                                                                                        W                  ⁡                                      (                                          n                      /                      2                                        )                                                                                                W                  ⁡                                      (                                          n                      /                      2                                        )                                                                                                                        W                  ⁡                                      (                                          n                      /                      2                                        )                                                                                                W                  ⁡                                      (                                          n                      /                      2                                        )                                                                                                                    (        1        )            where W′ denotes the logical complement of W, and W(1)=|0|.
A Walsh sequence or code is one of the rows of a Walsh function matrix. A Walsh function matrix of order n contains n sequences, each of length n Walsh chips. A Walsh function matrix of order n (as well as other orthogonal functions of length n) has the property that over the interval of n bits, the cross-correlation between all the different sequences within the set is zero. Every sequence in the set differs from every other sequence in exactly half of its bits. It should also be noted that there is always one sequence containing all zeroes and that all the other sequences contain half ones and half zeroes.
In the system described in the '459 patent, the call signal begins as a 9600 bit per second information source which is then converted by a rate ½ forward error correction encoder to a 19,200 symbols per second output stream. Each call signal broadcast from a cell is covered with one of sixty-four orthogonal Walsh sequences, each sixty-four Walsh chips, or one symbol, in duration. Regardless of the symbol being covered, the orthogonality of all Walsh sequences ensures that all interference from other user signals in that cell are canceled out during symbol integration. The non-orthogonal interference from other cells limits capacity on the forward link.
Each base station in a CDMA system transmits in the same frequency band using the same PN sequence, but with a unique offset relative to an unshifted PN sequence aligned to a universal time reference. The PN spreading rate is the same as the Walsh cover rate, 1.2288 MHz, or 64 PN chips per symbol. In the preferred embodiment, each base station transmits a pilot reference. In the description of the present invention different information is transmitted on the I and Q channels which increases the capacity of the system.
The pilot channel is a “beacon” transmitting a constant zero symbol and spread with the same I and Q PN sequences used by the traffic bearing signals. In the exemplary embodiment, the pilot channel is covered with the all zero Walsh sequence 0. During initial system acquisition the mobile searches all possible shifts of the PN sequence and once it has found a base station's pilot, it can then synchronize itself to system time. As detailed below, the pilot plays a fundamental role in the mobile demodulator rake receiver architecture well beyond its use in initial synchronization.
FIG. 2 depicts a generic rake receiver demodulator 10 for receiving and demodulating the forward link signal 20 arriving at the antenna 18. The analog transmitter and receiver 16 contain a QPSK downconverter chain that outputs digitized I and Q channel samples 32 at baseband. The sampling clock, CHIPX8 40, used to digitize the receive waveform, is derived from a voltage controlled temperature compensated local oscillator (TCXO).
The demodulator 10 is supervised by a microprocessor 30 through the databus 34. Within the demodulator, the I and Q samples 32 are provided to a plurality of fingers 12a-c and a searcher 14. The searcher 14 searches out windows of offsets likely to contain multipath signal peaks suitable for assignment of fingers 12a-c. For each offset in the search window, the searcher 14 reports the pilot energy it found at that offset to the microprocessor. The fingers 12a-c are then surveyed, and those unassigned or tracking weaker paths are assigned by the microprocessor 30 to offsets containing stronger paths identified by searcher 14.
Once a finger 12a-c has locked onto the multipath signal at its assigned offset it then tracks that path on its own until the path fades away or until it is reassigned using its internal time tracking loop. This finger time tracking loop measures energy on either side of the peak at the offset at which the finger is currently demodulating. The difference between these energies forms a metric which is then filtered and integrated.
The output of the integrator controls a decimator that selects one of the input samples over a chip interval to use in demodulation. If a peak moves, the finger adjusts its decimator position to move with it. The decimated sample stream is then despread with the PN sequence consistent with the offset to which the finger is assigned. The despread I and Q samples are summed over a symbol to produce a pilot vector (PI, PQ). These same despread I and Q samples are Walsh uncovered using the Walsh code assignment unique to the mobile user and the uncovered, despread I and Q samples are summed over a symbol to produce a symbol data vector (DI, DQ). The dot product operator is defined asP(n)·D(n)=PI(n)DI(n)+PQ(n)DQ(n),  (2)where PI(n) and PQ(n) are respectively the I and Q components of the pilot vector P for symbol n and DI(n) and DQ(n) are respectively the I and Q components of the data vector D for symbol n.
Since the pilot signal vector is much stronger than the data signal vector it can be used as an accurate phase reference for coherent demodulation; the dot product computes the magnitude of the data vector component in phase with the pilot vector. As described in U.S. Pat. No. 5,506,865, entitled “PILOT CARRIER DOT PRODUCT CIRCUIT” and assigned to the assignee of the present invention, the dot product weights the finger contributions for efficient combining, in effect scaling each finger symbol output 42a-c by the relative strength of the pilot being received by that finger. Thus the dot product performs the dual role of both phase projection and finger symbol weighting needed in a coherent rake receiver demodulator.
Each finger has a lock detector circuit that masks the symbol output to the combiner 42 if its long term average energy does not exceed a minimum threshold. This ensures that only fingers tracking a reliable path will contribute to the combined output, thus enhancing demodulator performance.
Due to the relative difference in arrival times of the paths to which each finger 12a-c is assigned, each finger 12a-c has a deskew buffer that aligns the finger symbol streams 42a-c so that the symbol combiner 22 can sum them together to produce a “soft decision” demodulated symbol. This symbol is weighted by the confidence that it correctly identifies the originally transmitted symbol. The symbols are sent to a deinterleaver/decoder circuit 28 that first frame deinterleaves and then forward error correction decodes the symbol stream using the maximum likelihood Viterbi algorithm. The decoded data is then made available to the microprocessor 30 or to other components, such as a speech vocoder, for further processing.
To demodulate correctly, a mechanism is needed to align the local oscillator frequency with the clock used at the cell to modulate the data. Each finger makes an estimate of the frequency error by measuring the rotation rate of the pilot vector in QPSK I, Q space using the cross product vector operator:P(n)XP(n−1)=PI(n)PQ(n−1)−PI(n−1)PQ(n)  (3)
The frequency error estimates from each finger 44a-c are combined and integrated in frequency error combiner 26. The integrator output, LO_ADJ 36, is then fed to the voltage control of the TCXO in the analog transmitter and receiver 16 to adjust the clock frequency of the CHIPX8 clock 40, thus providing a closed loop mechanism for compensating for the frequency error of the local oscillator. Fingers 12a-c are coupled to power combiner 24 which outputs a transmit gain signal 38 to analog transmit and receiver 16.
As described above, in current demodulator structures, a path must differ by at least one PN chip to have a separate finger allocated to its demodulation. However, there are cases when paths differ by less than a PN chip interval in the time, this situation leads to the existence of a “fat path.” Under traditional demodulator implementations, only one finger could be allocated to demodulate the fat path. One of the reasons for this is that once assigned to a path, the finger tracks the path movement independently. Without central coordination of the fingers multiple fingers will converge to the same peak of the fat path. In addition, the searcher tends to get confused when paths are tracked which are to close to one another.
On an orthogonal forward link, there is a great deal of energy in each of the paths because all of the energy from the base station to all mobiles is transmitted using the same PN offset which are channelized by use of orthogonal code sequences. Moreover, orthogonal code sequences have poor autocorrelation in that the correlation between orthogonal code sequences is high. Thus, when paths on the forward link differ by less than a PN chip interval, the signals cannot be distinguished from one another by the outer PN spreading nor is the coding gain of the orthogonal spreading realized because of the time shift. The energy of the close multipath components in this case serves as noise and substantially degrades the performance of the demodulator assigned to the fat path. On the reverse link, close multipath components can also cause degradation of the demodulator assigned to the fat path.
The present invention is described with respect to the improvement of the demodulation of the forward link. However, the present invention is equally applicable to improving the demodulation of the reverse link.